1. Field of the Invention
The present invention relates to a method for machining a workpiece employing a machine tool comprising a first chuck device for clamping one end part of a workpiece and a second chuck device for clamping the other end part of the workpiece, and more particularly to a method for machining a workpiece suitable for application in a crankshaft production line.
2. Description of the Related Art
Solid-type crankshafts which serve as a machine component part of an engine are normally manufactured by cutting and grinding a forged crankshaft starting material manufactured by employing machine tools, and the machining and manufacture of a crankshaft of a complex shape necessitates the implementation of a large number of machining steps employing a range of machining tools.
As is well known, a common crankshaft S as shown in FIG. 20 comprises a predetermined number of arms (crank arms) A1 to A8 to which journals (main journals) J1 to J5, pins (crank pins) P1 to P4 and balance weights are integrally coupled, a front shaft F to which a timing pulley or tensional damper or the like is affixed being formed in a front-side end part (left-end part in the drawing), and a rear flange (output-side flange) R to which a flywheel or the like is affixed being formed in a rear-side end part (right-end part in the drawing) thereof.
A production line for carrying out machining steps from a forged crankshaft starting material to a semi-finishing machining step of the pins P1 to P4 is normally configured from a total of six machining tools comprising, as shown in FIG. 21, a centering machine M1, a front-side lathe M2, a rear-side lathe M3, and a first crankshaft mirror (pin mirror) M4, a turn-turn broaching tool M5 and a second crankshaft mirror (pin mirror) M6 of a shaft machining device, the crankshaft S being manufactured to a predetermined shape as a result of a crankshaft starting material S′ being successively introduced into and machined by the machine tools M1 to M6.
That is to say, first, the centering machine M1 performs a planar machining of the front-side end face and rear-side end face of the crankshaft starting material S′ and forms center holes in each of the front-side end face and rear-side end face.
Next, the front-side lathe M2 performs a lathe-turning machining of the outer circumferential surface of the front shaft F and journal J1 and the external shape surface of the arm A1, after which the rear-side lathe M3 performs a lathe-turning machining of the outer circumferential surface and external shape surface of the rear flange R.
The machining steps performed by the front-side lathe M2 and rear-side lathe M3 constitute steps that involve forming of a clamp part in each of the front side and rear side of the crankshaft starting material S′.
Next, the first crankshaft mirror (pin mirror) M4 performs a rough-surface machining of the outer circumferential surface of the journals J2 to J5, the outer circumferential surface of the pins P1 to P4 and the external shape surface and inner surface of the arm A1, and performs a rough-surface machining of the external shape surface and inner surface of the arms A2 to A8.
Next, the turn-turn broaching machine M5 performs a semi-finish machining of the outer circumferential surface of the journals J2 to J5, after which the second crankshaft mirror (pin mirror) M6 performs a semi-finish machining and groove-machining of the outer circumferential surface of the pins P1 to P4.
However, the large number (six in total) of machine tools for which equipment is required of a conventional production line such as the one described above carries an inherent problem of very high installation costs associated with construction of the production line (initial establishment costs) and running costs associated with actual operation of the production line.
That is to say, there are unavoidable equipment costs associated with the six machine tools and the loaders provided between the machine tools in the construction of the production line and, furthermore, there are increased land costs as well as increased fundamental construction costs associated with the greater space required for securing the factory building area for provision of the production line.
In addition, a large number of machine tools (six in total) means a large quantity of electrical power energy is consumed in the operation of the production line and, in addition, apart from the costs necessitated by the processing of the scraps and so on discharged from each of the machining tools, there are unavoidable upkeep costs associated with maintenance and inspection of each machine tool.